Recently, it is required that the user terminal is to become smaller, cheaper and consume less power according to the wide use of the portable wireless telephone. Thus, direct conversion receiver is broadly used. If direct conversion receiver is used, the channel filter of bandwidth pass filter (BPF) which is necessary to the superheterodine receiver is not required since there is no image frequency component. Instead of that, low pass filter (LPF) is used as the channel filter. Therefore, the integrated circuit fabrication process can be made simple.
Direct conversion receiver basically transforms the input radio frequency signal to the baseband frequency signal directly. This corresponds to the case that the middle frequency is 0 Hz for the superheterodine receiver. Therefore, local oscillator signal having nearly the same frequency as the input frequency is generated and mixed with the input frequency to directly transform the input radio frequency signal to the baseband frequency signal. FIG. 1 shows the input signal whose carrier frequency is ωRF, LO signal whose frequency is ωLO, and the baseband signal generated by mixing the above two signals in the frequency domain. FIG. 2 is a circuit diagram illustrating the mixer 205 for mixing the input signal 201 and LO signal 203 and related circuits.
As shown in FIG. 1, the middle frequency ωIF signal having zero (0) Hz frequency is generated by mixing the LO signal having the same frequency ωLO as the carrier frequency ωRF of the RF modification signal using a mixer. The middle frequency signal whose frequency is 0 Hz generated by mixing the carrier frequency and LO signal includes the baseband frequency signal, LO frequency ωLO signal, carrier frequency ωIF signal, the signal whose frequency ωLO+ωIF is the sum of the LO frequency and the carrier frequency, and the signal whose frequency |ωLO−ωIF| is the difference between the LO frequency and the carrier frequency components. It is possible to extract the information signal and the signal whose frequency |ωLO−ωIF| is the difference between the LO frequency and the carrier frequency component by cutting off the signals except the baseband frequency signal among those components using low pass filter as shown in FIG. 2.
This direct conversion receiver still has some shortcomings as listed below.
First, the direct conversion receiver has the problem of DC offset voltage due to the mixing of the signals having the same frequency,
The reason of the generation of the DC offset is as follows. As shown in FIG. 2, a part of the LO signal is leaked through the radio frequency signal input terminal of the mixer 205 due to the characteristic of the hardware. The leaked LO signal is reflected from the output port of the radio frequency signal amplifier 209 and the antenna 211. The reflected LO signal is mixed with the LO signal in the mixer and appears as the DC component in the output signal. This is called as dynamic DC offset. The amount of the dynamic DC offset depends largely on the reflective ratio at the radio frequency signal amplifier 209 and the antenna 211. The reflective ratio is variable according to the amplifying ratio of the amplifier 209, and therefore, the amount of dynamic DC offset is variable to time.
Another reason for DC offset is non-symmetric characteristic between circuits contributes to the signals of positive phase and the reverse phase in the case of so-called balanced mixer which obtains the signals of positive and reverse phases from the signal component whose frequency |ωLO−ωIF| is the difference between the LO frequency and the carrier frequency component. Then, the signals of positive phase and the reverse phase from the signal component whose frequency |ωLO−ωIF| is the difference between the LO frequency and the carrier frequency component are generated asymmetrically and DC offset is brought about. This is called as static DC offset.
Those DC offset remains in the output signal as the non-preferable component, and becomes a reason of deterioration of the characteristics of the direct conversion receiver using mixer. Therefore, the DC offset voltage existing in the mixer should be removed.
Second, the direct conversion receiver has a problem of second order intermodulation (IMD2) component. Most mixer circuits are implemented by the active elements. The active elements show non-linear characteristic especially in the case that the radio frequency signal is inputted.
This non-linear characteristic can be approximated by the power series. The most important component of the even order component among the components which is approximated by the power series is the second order component, and the even order component can be approximated as the second order component.
However, the non-linear characteristic of the mixer circuit as described above is known as the main reason for deterioration of the performance of the whole circuit of the direct receiver.